Engine air-fuel ratio may be maintained at a desired level (e.g., stoichiometric) in order to provide desired catalyst performance and reduced emissions. Typical feedback air-fuel ratio control includes monitoring of exhaust gas oxygen concentration by an exhaust sensor(s) and adjusting fuel and/or charge air parameters to meet a target air-fuel ratio. However, such feedback control may overlook cylinder-to-cylinder variation in air-fuel ratio, which may degrade engine performance and emissions. While various approaches have been set forth for individual cylinder air/fuel control, with the aim at reducing cylinder to cylinder air/fuel ratio variation, such variation may still persist as recognized by the inventors herein. For example, issues with cylinder air/fuel ratio imbalance may include increased NOx, CO, and hydrocarbon emissions, knocking, poor combustion, and decreased fuel economy.
Example methods to address air/fuel ratio imbalance include counting air/fuel ratio deviations from an expected value when applying an air-fuel ratio dither control. One example of a counting approach is shown by Hasegawa et al. in U.S. Pat. No. 7,721,591. Therein, when the number of times that deviation of a particular cylinder's air/fuel ratio exceeds a threshold is greater than a predetermined number, it is concluded that an abnormality occurs in that cylinder of the engine. Another approach for air/fuel ratio imbalance determination includes a differential summation method, which samples exhaust gas sensor signal differential values (e.g., lengths) at frequencies corresponding to the engine firing frequency or twice engine firing frequency, and calculates a summation of those values that are greater than a noise rejection floor threshold. The summation is then compared to a failure threshold value for determining cylinder air-fuel imbalance.
However, the inventors herein have also recognized potential issues with such systems. As one example, the approach described in Hasegawa relies on air-fuel ratio dither control to differentiate the air-fuel ratio of each individual cylinder. The dither control purposely varies the air-fuel ratio of a given cylinder. Such air-fuel ratio control is intrusive and may result in degraded emissions and/or over-fueling events. In another example, the summation method may suffer from sampling variation of differential signal lengths that leads to inconsistent results. Further, many of the differential signal lengths collected in the summation method may fall into a sensor signal noise band, where fault separation between small and large deviations from stoichiometric air-fuel ratio is low. To compensate for this, the noise floor threshold may be increased, leading to a diminished sample size and unreliable results. Another limitation to the summation method is its constriction to higher vehicle loads.
In one example, the issues described above may be addressed by a method for adjusting engine operation responsive to cylinder air/fuel imbalance. The imbalance is determined based on a total number of instances where sensed peak-to-peak exhaust lambda differentials are less than a threshold normalized to a total number of peak-to-peak oscillations. In this way, cylinder air/fuel imbalance may be monitored non-intrusively during normal engine operation by counting instances where peak-to-peak lambda differentials are below a threshold (e.g., are not indicative of cylinder imbalance) and comparing the counted instances against an imbalance threshold. By analyzing differentials that fall below a noise threshold, the number of samples counted may be increased and a greater fault separation may be provided.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.